Continuously variable transmissions (CVTs) are a result of the continuing pursuit of more fuel-efficient motor vehicles. The CVT differs from the conventional automatic transmission in that automatic transmissions use planetary gear sets to accomplish speed ratio changes, whereas CVTs use pulleys and a belt to change speed ratio. A conventional automatic transmission usually offers four, five or six fixed ratios or speeds, whereas a CVT offers an infinite number of ratios which can be achieved by changing the relative radius of travel of the driving belt on the driving and driven pulleys.
The variator of the CVT includes two steel pulleys and a steel belt. Each pulley is comprised of two opposing angled sheaves, one moveable and one fixed. The pulleys can be opened and closed to allow the belt to travel at different radii by axial movement of the moveable sheave with respect to the fixed sheave. When the driving pulley is fully open (small radius of belt travel) and the driven pulley is fully closed (large radius of belt travel) very high speed reduction ratios are achieved. Conversely, when the driving pulley is fully closed (large radius of belt travel) and the driven pulley is fully open (small radius of belt travel) increases in output speed over input speed are achieved.
CVTs have become increasingly popular in recent years because they may provide improved fuel economy versus conventional step gear automatic transmissions, the ability to operate the engine at lower rpms over a wider range of the fuel economy schedule, smooth shifting, more efficient vehicle front end packaging, as well as manual transmission interchangeability and all-wheel drive compatibility. The application of CVTs into light duty vehicles, especially for future use in conjunction with vehicles having higher horse power engines, requires CVTs to have higher torque capacity and excellent durability or wear resistance of components.
A problem which can occur with current designs results from internal belt vibrations and can manifest itself as “belt shudder” or “scratch.” Belt shudder is an objectionable vibration which has been found in certain CVT arrangements. This type of vibration begins to occur at relatively low mileages and can be commercially unacceptable. Belt shudder usually occurs during light throttle parking lot maneuvers, i.e., conditions under which less than ten percent throttle is applied, when the speed is in the range of 1400 to 2200 rpm. It usually occurs at a temperature below 80° C. Belt shudder is caused by a stick-slip phenomenon that occurs between the inner band and the element shoulder of the CVT belt. Belt shudder usually occurs when the crosshatched surface of the inner band becomes polished down as a result of wear. The belt shudder occurs at the transition point at which the belt changes from a pull to a push torque transfer mechanism.
The two primary methods for reducing the incidence of belt shudder are calibration strategy and fluid development. Calibration helps avoid regions or conditions under which belt shudder occurs. Proper calibration reduces pressure during transition from push to pull to “ratio-around” the problem. Fluid with properly designed friction characteristics can help prevent stick-slip phenomenon. The desired friction characteristics include positively sloped friction coefficient versus sliding speed, or higher dynamic friction than static friction. This represents a significant technical challenge when trying to balance against the needs of other transmission requirements.